1. Field of the Invention
The present invention relates to a powertrain and a method of controlling the same, and in particular, to a technique of controlling a powertrain for a vehicle having an engine and a rotating electric machine as motive power sources.
2. Description of the Background Art
Conventionally, a hybrid vehicle having an internal combustion engine and a rotating electric machine as motive power sources is known. In such a hybrid vehicle, at least one of the internal combustion engine and the rotating electric machine is used in accordance with a traveling state of the vehicle. For example, the internal combustion engine is mainly used for traveling at a high speed, and the rotating electric machine is mainly used for traveling at an intermediate or low speed. One such hybrid vehicle is configured to allow a differential mechanism having an engine and an electric rotating machine coupled thereto to function as a continuously variable transmission.
For example, Japanese Patent Laying-Open Nos. 2006-046487 and 2006-017232 disclose a vehicular drive apparatus transmitting an output of a drive force source to driving wheels via two shift mechanisms, which are: a continuous shift portion constituted of the aforementioned differential mechanism; and a stepwise shift portion provided in a stage behind the continuous shift portion.
In particular, Japanese Patent Laying-Open No. 2006-046487 provides a control apparatus that changes, when one of the two shift mechanisms has entered a failure state where a normal operation is not possible, a gear ratio of the other shift mechanism so that a total gear ratio, which is implemented based on a gear ratio of the continuous shift portion and a gear ratio of the stepwise shift portion, is attained in the value immediately before the failure of the one shift mechanism. With the control apparatus, the traveling performance of the vehicle can be ensured even if one of the two shift mechanisms has failed.
Japanese Patent Laying-Open No. 2006-017232 discloses switching an operation of the continuous shift portion to a stepwise shift state when an electric motor for allowing the differential mechanism to function as a shift mechanism has entered a state where a normal operation is not possible, thereby ensuring appropriate traveling performance of the vehicle.
According to the powertrain disclosed in Japanese Patent Laying-Open Nos. 2006-046487 and 2006-017232, the shift mechanism is configured to include a plurality of engagement elements (clutches and brakes) that are engaged or disengaged by a hydraulic actuator. Accordingly, in a high-load traveling of the vehicle, that is, when the engine or the rotating electric machine as a drive force source is outputting torque, if a situation where a supply hydraulic pressure to a stepwise shift portion is reduced occurs, each engagement element is disengaged, whereby each rotary element rotates at a high speed. Thus, durability of the electric motor or constituent components of the shift mechanism may possibly be deteriorated. When the shift mechanism is formed with a CVT (continuously variable transmission) also, the problem of high-speed rotation of the rotary elements associated with a reduction in the supply hydraulic pressure similarly occurs.